1. Field of the Invention
This invention relates to the field of electronically tunable optical filters utilizing noncollinear acousto-optic interaction in a birefringent crystal.
2. Description of Prior Art
Electronically tunable optical filters have been constructed so that a cone of light of a first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first to a second polarization of the light beam for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of tunable acousto-optic filters (TAOF) have been constructed: collinear and noncollinear. In the collinear filter, the incident and diffracted light beams inside the birefringent crystal are collinear with the acoustic beam. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters" appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288 entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the light beams inside the birefringent crystal are noncollinear with the acoustic beam. The diffracted light beam at the passband is selected from the incident light beam by either crossed polarizers or spatial separation. The noncollinear type of acousto-optic filter is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture" appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter."
In both the collinear and noncollinear types of tunable acousto-optic filters, the acousto-optic diffraction occurs for a narrowband of optical frequencies satisfying the relation that the sum of the momentum vectors of the incident light waves and acoustic waves equal the momentum vector of the diffracted light wave. More significantly, this narrow filter bandpass can be maintained for incident light having a distribution of incident directions. This large angular aperture characteristic is due to the proper choice of acousto-optic interaction geometry wherein the tangents to the loci of incident and diffracted light wavevectors are parallel. When the "parallel tangents" condition is met, the acousto-optic diffraction becomes relatively insensitive to the angle-of-light incidence, a process that is referred to as "noncritical phase matching." For the remainder of this disclosure, "tunable acousto-optic filter" or the abbreviation "TAOF" will be defined as an optical filter that operates on the basis of acousto-optic diffraction in a birefringent crystal wherein the noncritical phase matching condition is satisfied. This type of tunable acousto-optic filter is clearly distinguishable from the type with small angular aperture as described in an article entitled "New Noncollinear Acousto-Optic Tunable Filter Using Birefringence in Paratellurite" appearing on pages 256-257 of the Mar. 15, 1974 issue of Applied Physics Letters (Vol. 24), and in U.S. Pat. No. 3,953,107 entitled "Acousto-Optic Filters." The latter device is basically a narrowband deflector rather than a filter since its use must be restricted to well collimated light sources.
A preferred configuration of the noncollinear TAOF is disclosed in U.S. Pat. No. 4,342,502 entitled "Transverse Tunable Acousto-Optic Filter." The transverse configuration has the significant features of increased optical aperture and minimized optical aberrations. Practical implementation of the TAOF including, in particular, the transverse configuration has been hindered due to difficulties in the fabrication and impedance matching of acoustical transducers. The problem is particularly severe in the construction of tunable acousto-optic filters for realizing high spectral resolution.